Orthogonal code-division multiple access (OCDMA) has been proposed (U.S. patent application Ser. No. 980,957 now U.S. Pat. No. 5,375,140, incorporated herein by reference, and U.S. patent application Ser. No. 08/257,324 filed Jun. 7, 1994, incorporated herein by reference) as an effective technique for improving the capacity, i.e., bandwidth efficiency, of the more conventional quasi-orthogonal CDMA (QOCDMA). QOCDMA is frequently referred to as asynchronous CDMA since the chips (for direct sequence PN) or the hops (for pseudo-random hopping) are not aligned in time. As a consequence of the non-alignment, or asynchronicity, it is not possible to achieve true orthogonality and QOCDMA systems suffer from interference from other system users or what is commonly known as access noise. As a result of this access noise QOCDMA systems cannot achieve as high a capacity, i.e., number of channels, in a given amount of bandwidth as can an orthogonal system such as OCDMA.
As noted above, OCDMA requires time base alignment of all accesses and this can be done readily in star networks provided that the subscriber terminals, i.e., remote units, are not moving too quickly nor are too far removed from the base station, i.e., hub of the star network. In addition, there is another propagation condition that must be met that is of particular concern when the subscriber terminals employ antennas that do not have a great deal of directivity. In such situations there may be considerable multipath and the delayed path signals will contribute access noise since these signals being non-aligned in time will be non-orthogonal. However, if the multipath delay spread is small in comparison to the chip duration of the orthogonal codes, the impact of the multipath signals is negligible. By choosing a suitably low chipping rate and restricting operation to indoor environments, where the delay spread is quite small, this condition may be assured. Thus, the first application of OCDMA has been to a wireless PBX where these conditions can be met.
The object of the present invention is to extend this system to those environments where there is a significant delay spread in comparison to the duration of the orthogonal code chip duration.